The present invention relates to indirect lighting fixtures and system generally, and particularly to luminaires for indirect lighting which employ lens elements or other light transmissive media as a visible source of low brightness to persons in indirect lighting environments.
Indirect lighting, which is produced by reflecting light from a light source off a reflective surface such as a wall or ceiling, has long been criticized as producing a dull lighting environment, sometimes referred to as a cloudy day or funeral parlor effect. Nonetheless, lighting designers are increasingly considering indirect lighting systems for various applications because of the even illumination they provide and because they eliminate glare associated with direct lighting systems. Indirect lighting has become particularly advantageous in the open office environment where video display terminals (VDT's) are now prevalent and where uncomfortable glare on VDT screens often produced by direct lighting fixtures, such as the ubiquitous recessed ceiling fixtures (called troffers), can lead to VDT operator fatigue and, some now believe, long term health problems.
To overcome the perceived dull lighting environment produced by conventional indirect lighting, indirect fixtures have been devised with visible low brightness lens elements which give the observer of the fixture a perception or illusion of seeing the actual source of light. Such a fixture is described in U.S. Pat. No. 4,390,930 issued June 23, 1983, which discloses a linear extruded fixture for indirect lighting having lens strips running along the top of the fixture housing's opaque side walls. The lens extensions of the housing side walls have a prismatic surface formed to direct a small portion of the light received from the fixture's light source into normal viewing angles below the plane of the fixture. The resulting brightness of the visible lens surface is generally sufficient to give a perception of source brightness and as a result gives the psychologically more pleasing effect of being able to visually locate the light source, while avoiding discomfort associated with excessive brightness and glare producing contrast brightness.
Studies have shown a further psychological advantage to low brightness lens elements on indirect fixtures and particularly linear indirect fluorescent fixtures. It has been found that the low brightness lens elements below certain maximum brightness ranges will actually tend to increase a subject's perception of the overall light level in a space being illuminated by lensed indirect fixtures. Accordingly, lensed indirect fixtures will permit comfortable lighting at lower light levels resulting in lower energy consumption.
Despite its advantages, lensed indirect fluorescent lighting has heretofore suffered from the difficult problem of achieving uniform brightness in the visible fixture lenses. The principal problems are first the appearance of socket shadows on the surfaces of the lens elements adjacent the electrical socket holders for the fixture's fluorescent lamps, and secondly, uncontrollable brightness on particular prism surfaces at particular viewing angles. Socket shadows and localized areas of excessive brightness have become a particular problem with the advent of biax fluorescent lamps which are considerably smaller and have higher light output than standard sized fluorescent.
Excessive brightness in lensed indirect fixtures frequently occurs along the lens' very top edge. It can also appear within the body of the lens such as discussed in U.S. Pat. No. 4,698,734 issued Oct. 6, 1987, which addresses the problem of side angle lens brightness, that is, hot spots on the lens produced by prior prismatic lens designs at viewing angles other than a viewing angle that is perpendicular to the lens surface. The above patent discloses a solution to the side angle brightness problem using a lens design which to some degree sacrifices the lens' ability to spread the light overhead the fixture.
Still a further problem with lensed indirect lighting fixtures is the ability to control the overall lens brightness at normal viewing angles (roughly from near horizontal to 45 degrees below horizontal) at very low luminance levels. As above-mentioned, studies have found that a low brightness lens element on a lensed indirect fixture can increase the perceived light level in the indirectly lit environment. Generally, it is believed that lens brightness levels below approximately 400 footlamberts will provide a visually comfortable lens, however, ideally the lens brightness levels should be kept within lower brightness ranges of approximately 50-200 footlamberts. With existing lens indirect lighting fixtures, such low luminance levels are very difficult to achieve with uniformity over the lens surface.
The present invention overcomes the above-mentioned problems associated with lensed indirect lighting by providing an indirect lighting system having lens elements capable of being maintained at very low brightness levels and having nearly absolute uniformity of brightness over the entire observable lens surface. The present invention eliminates distracting socket shadows on the lens and produces a lens brightness level that tends to be self-adjusting with respect to the brightness levels on the overhead ceiling or upper wall surfaces adjacent to which the lighting fixtures of the system are suspended or mounted. That is, as the distance between the fixture and the reflective surface behind the fixture is increased, both the brightness of the background reflective surface and the fixture lenses decrease. The reverse is true if the fixture to surface separation is decreased.